According to what method is 3D cursor's location in 3D View determined when left clicking?
The 3D cursor moves in the plane facing the view you are in and intersecting its previous position.

Is you set up this configuration : a camera (bottom right), a plane (selected) 90° intersecting the camera view, you can notice this behaviour.
Go in the camera view using 0, click somewhere to move the 3D cursor, and come back to the top view. The cursor will stay on the plane.



The distance of the plane is defined by the normal vector N of the view (N normalized) dot the camera-"prev cursor position" vector. Or if C is the camera, P the previous position, the distance is ||N.CP||